The present invention relates to a system for recording/reproducing a charge latent image including a recording medium used therein.
In compliance with an increased demand in recent years for a reproduced image with high picture quality and high resolution various systems such as a so-called EDTV (Extended Definition TV) system, or an HDTV (High Definition TV) system have been proposed for a television system. In order to obtain a reproduced image with high picture quality and high resolution, it is required to provide an image pickup device capable of producing a video signal from which a high picture quality and high resolution image can be recreated. However, for conventional image pickup devices using an image pickup tube, it is difficult to generate such a video signal. The reasons for this are as follows: Since there is a limit to the reduction of the diameter of an electron beam in the pickup tube, high resolution image reproduction by reduction of the diameter of the electron beam cannot be expected. Alternatively, if the target area of the pickup tube is increased, the level of the output signal will be reduced because of the increased output capacity which is proportional to the area of the target. Therefore, high resolution image reproduction by the increase of the target area cannot be realized. Furthermore, in the case of an image pickup device for a moving picture, since the frequency range of such a video signal reaches several tens and several hundreds MHz for implementation of the high resolution image, the increase of output capacity, i.e. the increase of target area, is not preferable.
On the other hand, an increase of picels or a downsizing of a pixel of solid state image sensors has produced difficulties known to the industry.
As stated above, conventional image pickup devices of either a pickup tube or a solid state sensor could not satisfactorily generate such a video signal to provide a reproduced image of high picture quality and high resolution because of the inevitable use of an image sensor in the construction thereof. In order to solve the problem, the assignee of this application has already proposed an imaging system and a recording system to obtain a high resolution optical image by an image pickup device using a photo-to-photo transducer, and to record such an optical image as a charge image of high resolution onto a charge accumulation layer (or a charge hold layer) by using a photo-to-charge transducer.
Naturally, in the implementation of such an imaging system and recording system, a comparably capable read out system is required which reproduces such a charge image recorded on a recording medium as an electric signal. However, no satisfactory reproducing apparatus for this has been available to data.
In the use of the charge latent image, conventional technology has been disclosed in European Patent Application (EPA) No. 89306243.0 (Publication No. 0 348 162 A2) or EPA. No. 87311531.5 (Publication No. 0 273 773 A2).
FIG. 1 shows a schematic arrangement of a recording unit of a prior art system. The recording unit comprises an imaging lens 1 as an imaging means for forming an optical image of an object 0; a recording head 2 consisting of a transparent electrode 3 and a photo-conductive layer (hereinafter abbreviated in a PCL) member 4; a recording medium 5 including a carrier transport layer (CTL) 6 of a hole transport type (HTL) or an electron transport type (ETL), a fine-grained layer 7 formed from a photo-conductive grain (PCG), a dielectric layer (IL) 8, and an electrode 9; a power source 10 for supplying electric power having a specified polarity to the electrodes 3 and 9; and high source 11 for generating an electron-hole pair in the PCG of the fine grained layer 7 by irradiating a light to the medium 5.
The PCG layer 7 is disposed between the CTL 6 and the IL 8. The carrier transport layer 6 is a layer of a compound known to the industry. A layer of such a compound having a high hole mobility is called a hole transport type layer (HTL), and one having a high electron mobility is called an electron transport type layer (ETL).
Depending on the desired charge polarity of the charge latent image eventually recorded in the recording medium 5, either the HTL or the ETL is used for the carrier transport layer 6, i.e., when the HTL is used, the polarity of the charge latent image will be negative, and in the case of the ETL, a positive polarity charge latent image will eventually be left recorded in the recording member 5.
However, since the above technology records the charge of a latent image, which corresponds to charge distribution according to a form of the optical image, on the surface of the recording medium, there is a problem that a preserved condition of charge latent information is deteriorated on the basis of the decrease and change of the recorded charge quantity because of changes in the physical condition during storage and reproducing frequency.
Furthermore, even if there is no problem of resolution for the first time of reproducing the recorded charge latent image, when new charge information is recorded again once after erasing recorded charge information from the medium recording the charge latent image, is a problem that the resolution for reproducing the new latent image is sharply deteriorated in comparison with the former resolution.